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Learn more about psychoacoustics, science
that associates psychology with auditory perception

Tuning the sound of the machines,
from the blender to Rolls-Royce

LUIZ SUGIMOTO

Cthe day will come when that even the noise of the blender will become more pleasant, due to the industry's growing concern with acoustic comfort and the subjectivity of customers' sound assessment. Knowing why the sound of water dripping from the tap is irritating, while the sound of millions of raindrops increases the pleasure of falling asleep, is the question that underlies psychoacoustics, a relatively old science that associates psychology with auditory perception, in search of what in engineering jargon is called sound quality.

Professor José Roberto de França Arruda, from the Department of Computational Mechanics at the Faculty of Mechanical Engineering (FEM) at Unicamp, is involved with other colleagues in various projects, which aim to improve everything from musical instruments and room acoustics, to the noise level inside cars and aircraft. "Traditionally, the approach in the laboratory was to attenuate noise and manufacture a machine as silent as possible. In Brazil, the "noise seal" was instituted (1994), which must indicate the sound power level of every household appliance. Solving This basic problem comes with being careful with the impression that the sound makes on the user. For the purchase decision, the noise does not necessarily need to be low, but pleasant", explains Arruda.

Hence, the fact that psychoacoustics has its paradigm in music. "Music is the sound we want to hear, the pinnacle of sound quality. There are musical concepts that we try to bring to engineering, in order to know how a product should sound", adds the professor, who is in charge of the Laboratory of Vibroacoustics, where he prepares a project to develop new acoustic boxes that offer better sound directionality.

Jury tests - At the beginning of the last century, there are records of sound quality tests in the automobile industry. Car horns were installed on panels and sounded in front of a jury who chose those with the best sound. Juries were also used to evaluate the noise from passing cars, already indicating concern about noise pollution. "The jury test is typical of psychoacoustics and must be guided by psychological criteria. This is because the same noise sounds one way if the juror is calm and in a pleasant environment, and another way if the person is stressed. It is necessary to assess whether the judges are in a situation of certain neutrality", explains the researcher.

The tests are standardized, using an instrumented torso of microphones to capture sound pressure oscillations. The aim is to reproduce all the dynamics of the hearing aid and the influence of the presence of the human head and torso. Then, there is an equalization in sound reproduction. "Instead of putting several people in the car and going around so that everyone hears the same sound, what is done is recording the sound of the vehicle and playing it back in a controlled environment. Each juror puts on a headset and responds to a series of questions. These tests are frequent in the automobile industry", informs França Arruda.

Such systems allow sound editing. Once a harmonic component that makes vehicle noise unpleasant is identified, that frequency band can be removed. If the subjective quality improves, the problem is passed on to the mechanical engineer, who will locate the vibration causing that noise component and try to eliminate it. Another common solution is to increase the noise of another track to cover up the one you dislike, an effect called "masking". "Automakers have NVH sectors (noise, vibration and harshness, or noise, vibration and what can be translated as harshness). Based on laboratory analysis, engineers will look for a solution from the point of view of vibration or noise to achieve the sound quality target", explains the FEM professor.

Harley-Davidson - Investing in the NVH sector is more rewarding for the industry than manufacturing an expensive product, with high quality components and precise adjustments, as sound engineering has tools that allow achieving similar results in lower standard vehicles. Arruda admits, however, that the sound of a luxury car reflects the care with which it was manufactured: "People are already used to identifying the sound of quality. Engineering could, for example, make the door slam of a car popular car sounded like a Rolls-Royce, even if the customer didn't expect it. Now, if the Rolls-Royce door slam sounds like a popular car, the customer will complain."

Automakers have already gone so far as to define an engine noise associated with the brand, and can return to the supplier a batch of exhausts that do not meet their specifications. "In this aspect we have another paradigm, the legendary Harley-Davidson, which is celebrating 100 years of existence. It was the first company to patent the noise of an engine. No one can manufacture a motorcycle that imitates that sound, which has become a signature, the spirit of the brand. The problem for Harley engineers now is to ensure that the engine's roar is not distorted with the adoption of new technologies, which would be unacceptable for fans", concludes França Arruda.

Simulating eddies It's not that obvious. One of the main sources of noise from a moving plane may not be in the engines or turbines, but in the air flow around the outside of the structure. Aerodynamic pressures induce shock phenomena: on the wing, for example, air passes quickly and creates areas of eddies that cause the structure to vibrate. The coupling between aerodynamic noise and structural vibrations generates very high sound pressures inside aircraft. The noise generated would be unbearable for passengers if the aircraft did not have several layers underneath the metal structure mixed with different insulation materials, such as acoustic foams, glass fibers, blankets and air chambers. Also in the Department of Computational Mechanics, professor Renato Pavanello and postgraduate student Francisco Ilson da Silva Júnior design new computer programs that allow calculating thicknesses, shapes and distribution of properties of each of these materials. "We developed methods that can be applied to aircraft, automobiles, homes, studios, auditoriums. These are computer simulation tools that allow engineers to improve products and techniques from the point of view of acoustic comfort", explains Pavanello. Studying structural vibration on the one hand, the acoustic effect on the other, and in between the efficiency of insulating materials, would in principle be a simple task. More complex is the study of these phenomena in a coupled way, a key word in the work carried out by advisor and supervisee. "Topology, shape of bodies, mechanical properties such as density, porosity and tortuosity of poroelastic materials are some of the information computed", says Ilson da Silva Jr. The aircraft give a measure of the complexity of the tool developed at FEM, but it is with the automobile industry that privileged contact is maintained. According to Pavanello, a partnership with General Motors allowed its engineers to carry out postgraduate research in the laboratory. "GM has a very well-equipped proving ground in Indaiatuba, including a specific vibration and noise area, and could simply import software for vibroacoustic analysis. But, before applying these tools to real projects, its engineers seek out the GM team DMC to improve your knowledge. Our simulator is at the same level as those sold internationally", assures the professor. Pavanello also highlights the full mastery of the tool, as it was fully developed at FEM with current computer programming techniques, which makes it available to other researchers in the area. The program is being developed by a team of postgraduate students and has the decisive participation of professor Janito Vaqueiro Ferreira, who is a specialist in scientific computing. It is not yet possible to use the program in open, three-dimensional, infinite environments, such as simulating noise transmitted by vehicles to homes around a highway. "It is difficult to model a non-reflective border using finite dimensions. To do this, techniques are used that truncate the analysis space, trying to represent it as if it were the surroundings of a road. Regarding this, we already have several interconnected research", he informs the teacher. Although it is not a FEM project, anyone traveling along Rodovia dos Bandeirantes may notice acoustic barriers that protect a neighboring condominium upon arrival in São Paulo. "Part of the sound reflects off the barrier and another part passes over it. It is possible to considerably reduce noise in homes, as long as appropriate material is used and the position of the barriers is correct. Our software is capable of simulating this."

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